RFID (radio frequency identification) is one typical means of locally usable wireless communication. RFID is implemented as a system primarily made up of tags and a reader, the reader reading information off each tag in noncontact fashion. Also called an ID system or a data carrier system, RFID is a recognition system that utilizes radio frequencies (radio waves). Communication between a tag and a reader/writer is effected by diverse methods including electromagnetic coupling, electromagnetic induction, and radio frequency communication (refer to Non-Patent Document 1)
An RFID tag is a device that contains specific identification information. In operation, the tag generates radio waves at a modulation frequency corresponding to the identification information upon receipt of radio waves on a particular frequency. The reader having read the RFID tag can determine its identity based on its oscillation frequency. It follows that an RFID-based system allows goods carrying RFID tags or owners of the goods to be identified using particular IDs written in the tags. At present, RFID is applied to numerous systems such as entry/exit management systems for managing people's comings and goings into and out of a controlled room, article identification systems deployed in the distribution industry, bill settlement systems at cafeterias, and theft prevention systems at retailers handling CDs and software products.
Illustratively, it is possible to build an IC chip having capabilities for data transmission and reception as well as data storage, a power source for driving the chip, and an antenna into a small-sized wireless identification apparatus in packaged form (refer to Patent Document 1). With this wireless identification apparatus in use, various items of data on goods and articles are transmitted via the antenna to receiving means of the IC chip. The transmitted data can be stored in a memory of the chip and can be output wirelessly to the outside via the antenna. This makes it possible for goods or articles carrying the chip to be tracked down and identified for their presence and location.
FIG. 9 shows a typical configuration of a conventional RFID system. Reference numeral 101 denotes an RFID tag constituted by a tag chip 102 and an antenna 103. The antenna 103 may illustratively be a half-wave dipole antenna. The tag chip 102 is made up of a modulation unit 110, a rectification/demodulation unit 112, and a memory unit 113.
A radio wave f0 transmitted from a tag reader 100 is received by the antenna 103 before being input to the rectification/modulation unit 110. The unit 110 rectifies the received radio wave f0 into a direct current that triggers a demodulation function. The radio wave is thus recognized as a readout signal from the tag 101. The power generated by reception of the radio wave f0 is also fed to the memory unit 113 and modulation unit 110.
The memory unit 113 reads ID information previously stored internally and sends the retrieved information to the modulation unit 110 as outgoing data. The modulation unit 110, composed of a diode switch 111, turns on and off the switching action repeatedly in keeping with a bit image of the outgoing data. More specifically, when the data is “1,” the diode switch 111 is turned on to terminate the antenna at antenna impedance (e.g., at 50 ohms). At this point, the radio wave from the tag reader 100 is absorbed. When the data is “0,” the diode switch 111 is turned off (i.e., opened) to terminate the antenna in an open state. At this point, the radio wave from the tag reader 100 is reflected and sent back to where it came from. The reflection-absorption pattern of the incoming radio wave represents data in what is known as the back scattering system. In this manner, the tag 101 can transmit its internal information without dissipating power.
The tag reader 100 is constituted by a host device 106 such as a PDA, a tag reader module 104, and an antenna 105 connected to the tag reader module 104.
The host device 106 sends a read command from the tag 101 to a communication control unit 119 via a host interface unit 120. Upon receipt of the read command from the host interface unit 120, the communication control unit 119 edits outgoing data in a predetermined manner, filters the edited data, and sends the filtered data to an ASK modulation unit 117 as a base band signal. The ASK modulation unit 117 carries out ASK (amplitude shift keying) modulation by use of a frequency f0 of a frequency synthesizer 116.
The frequency synthesizer 116 sets the frequency under control of the communication control unit 119. Generally, the transmission frequency to an RF tag is determined through hopping so as to reduce standing waves and multipath interference in the signal coming from the tag. A hopping command is also given by the communication control unit 119. An outgoing signal having undergone ASK modulation is forwarded to a circulator 114 before being emitted to the tag 101 from the antenna 105.
The tag 101 returns a signal at the same frequency as that of the signal coming from the tag reader 100 by the effect of reflection through back scattering (as described above). The returned signal is received by the antenna 105 of the tag reader 100 and input to a mixer 115. Because the mixer 115 admits the same local frequency f0 as that of the outgoing signal, a signal modulated by the tag 101 appears at the output of the mixer 115. A demodulation unit 118 demodulates data formed by 1's and 0's out of the signal and forwards the data to the communication control unit 119. The communication control unit 119 decodes the data so as to acquire ID data that was held in the memory unit 113 inside the tag 101, and transfers the ID date through the host interface unit 120 to the host device 106.
In the above-described setup, the tag reader 100 can read information from inside the tag 101. The tag reader 100 generally doubles as a tag writer that can be used to write the data designated by the host device 106 into the memory unit 113 in the tag 101.
Conventionally, the above-described kind of back scattering type wireless communication system tags has been applied mainly to the identification and recognition of articles and persons typically through the use of RFID tags. That is because the range of communication by the communication system has been limited to relatively short distances.
Meanwhile, back scattering type wireless communication has the potential for providing wireless transmission channels at a very low level of power dissipation as long as the communication distance is limited. Recent years have witnessed the advent of IC chips carrying memory functions thanks to improved packaging techniques, with the memory capacity getting larger over time. Such developments have aroused the needs not only for the communication of identification and recognition information over relatively short distances but also for general data transmission applications based on back scattering type communication.
However, conventional back scattering type communications systems have failed to offer high data transmission rates necessary for general applications. That is because they operate on the principle of ASK (amplitude shift keying) or BPSK (binary phase shift keying), i.e., modulation methods of relatively low bit rates.
[Patent Document 1]
Japanese Patent Laid-open No. Hei 6-123773
[Non-Patent Document 1]
“RFID Handbook: the Principle and Applications of Noncontact IC Cards” (by Klaus Finkenzeller, translated into Japanese by Soft Kogaku Kenkyusho Ltd., and published by The Nikkan Kogyo Shimbun Ltd.).